1. Field of the Invention
The present invention relates to the processes of galvanizing and galvannealing a metal strip. Specifically, the present invention relates to a soak furnace capable of being used for after pot cooling in the galvanizing of a metal strip and for heat treatment of the zinc coated strip to complete the alloying in the galvannealing of a metal strip. The soak furnace allows for various adjustments in the soak time and temperature conditions of the strip in order to optimize the galvanneal coating phase compositions for a wide variety of steel grades.
2. Description of the Prior Art
In a galvannealing process, a zinc coating may be deposited on a steel strip. The zinc coated strip may then be heated in an alloying furnace in order to form a zinc alloy and then may be further heated in a soak furnace in order to complete the alloying process. In general, it is desirable for the galvannealed coating to include primarily a delta microstructure and avoid zeta and gamma phases. The greater the amount of gamma phase in the coating, the greater the chance that the coating will be too brittle, and the greater the amount of zeta phase in the coating, the more likely that the coating will be too soft. In general, excessive gamma phase may be formed when the strip is heat treated within the soak furnace for too long a time and/or at too high a temperature. Conversely, zeta phase may be formed when the strip soaks within a soak furnace at too short a time and/or at too low a temperature.
In order to optimize galvanneal coating phase composition for a variety of steel grades with a variety of coating thicknesses, one may optimize the soaking temperature and duration of the strip in the soaking environment. When the soak furnace is of a fixed length, generally it is not possible to adjust the soak duration without potential loss in productivity. Soaking furnaces without adequate supply of hot and cold air cannot maintain a desired thermal profile during the strip's transition through the furnace. Therefore, a soaking furnace capable of providing desired thermal environment for a desired time (duration) for substrates with different chemistries, different coating thicknesses and different process speeds is essential. This invention has been designed to overcome these shortcomings of soak furnaces with a fixed length and inadequate thermal atmosphere control.
U.S. Pat. No. 6,428,851 discloses a bath configured to allow for the thermal depositing of a coating onto a moving metal web. The process disclosed may be used for the priming of zinc and zinc-alloy coated steel webs. The disclosed process utilizes air nozzles to maintain the position and stability of the web as the web moves through a curing oven. Mist jets and blowers are used to cool the moving web prior to contacting a turner roll
Korean Patent Publication 2004055985 discloses a method for controlling the temperature and composition of atmospheric gas in the soaking zone of a galvannealing furnace. The disclosed method includes the steps of arranging atmospheric gas injection and sealing means on the inner lower side of a vertical soaking zone; passing mixed gas through a suction ejector; injecting the mixed gas using a blower; and injecting a second mixed gas into the soaking zone through a gas injection and sealing means. The first mixed gas comprises atmospheric gas and atmospheric composition adjusting gas, the latter previously mixed in intermediate step. A mixture of nitrogen and hydrogen or air may be used as the furnace atmosphere adjusting gas. The second mixed gas comprises first mixed circulation atmospheric gas and also combustion flue gas generated from a combustion chamber. The combustion chamber may be separately installed on the outside of the soaking zone. An air injection sealing means may be arranged on the upper part of the soaking zone, and the injection sealing means may suppress the outflow of atmospheric gas from an upper part of the soaking zone in order to cool the atmospheric gas and at the same time connect the air injection sealing means with the gas injection sealing means. According to this invention, the thermal soak profile is controlled by introducing cool gas in the lower part of the soak chamber and hotter gas in the upper part of the soak chamber to achieve the desired galvanneal powdering resistance. But the shortcoming of this method is that it cannot provide the flexible soak profile that is needed for a wide variety of steels because it cannot control the soak time at temperature due to the absence of separate soak zones divided by internal baffles.
Japanese Patent Publication 2003064421A generally discloses a processing apparatus for a steel strip in a continuous annealing furnace but not in a galvanneal soak furnace. The processing apparatus includes slidable baffle plates arranged on the right and left edges of the strip. The baffle plates alter the gap in the edges of the apparatus thereby varying the flow of coolant through the apparatus. The patent discloses arranging a pair of spray boxes in front of and behind a steel strip. The flow of coolant from the spray box is altered by adjusting the gap defined by the baffle plates. A difference in pressure may be generated with respect to the surfaces of the strip by adjusting the flow of the coolant. The baffle plates may be moved orthogonally with respect to the opposing surface of the spray boxes. In addition, the patent discloses that the spray box may be used to either cool or to dry the steel strip.
Japanese Patent Publication No. 2004307904A discloses a steel strip cooling device for a continuous annealing furnace but not for galvanneal soak furnace. The cooling device includes baffle plates arranged at predetermined intervals between projecting gas ejection nozzles connected to a pair of opposing cooling plates. The baffle plates may be arranged along the conveyance path of the steel strip. In addition, the cooling device may be used for a continuous annealing furnace and a zinc galvanizing furnace but not for galvanneal soak furnace. In addition, the device provides for the retention of gas near the edges of the steel strip and the flap of the steel strip, thereby improving the efficiency of the furnace.